1. Field of the Invention
The present invention is concerned with improvements in air-swept tube mills of the kind having at least two grinding chambers, a discharge chamber and an outlet which is preferably central, for instance through a hollow trunnion.
2. Descriptions of the Prior Art
In tube mills of the type contemplated, material is generally conveyed through the mill and out of the mill suspended more or less in an air current drawn or forced through the mill. The air current may be a cold or hot gas stream and may also serve the purpose of drying the material ground in the mill. Generally the finely ground material carried out of the mill suspended in the air current also comprises some coarse particles. In most instances it is necessary to separate off such coarse particles and return them to the mill for further grinding.
In a conventional grinding installation the grinding mill and the separator are generally individual units provided with tubes leading the airborne material from the mill to the separator from which the oversize material is usually returned to the mill inlet. However a manufacturing process according to this procedure is not always suitable for grinding mineral materials in raw materials for the manufacture of cement. In particular, when the raw material is moist and has to be dried in a drying chamber associated with the mill as a first treatment chamber succeeded by grinding chambers, it is difficult and uneconomical to mix the oversize material with the fresh material.
To solve this problem it has previously been suggested to divide the grinding process into two steps by utilizing mills having peripheral outlets through which the dried and preferably initially ground product is discharged from a first part of the mill and conveyed to a separator, whereupon the oversize material is returned to the second part of the mill in which the fine grinding takes place.
Further, it has been suggested to combine the grinding mill and the separator into one unit in which the mill is provided with screw conveyors or other means serving to return the oversize material to the first or second grinding chamber. However, the installation of returning means such as internal screw conveyors or the like involve a number of complications.
According to the invention in an air-swept tube mill having at least two grinding chambers and a discharge chamber, at least the second or last grinding chamber has an internal by-pass through which the first grinding chamber communicates with the discharge chamber, enabling ground material to by-pass the second or last grinding chamber. The second or last grinding chamber is arranged to receive material from the discharge chamber for grinding during passage of the material in a direction opposite that of the direction in which the material is conveyed through the preceding grinding chamber, the material being discharged into the air stream through the by-pass.
A feature of the present invention relates to the provision of a new mill design provided with means which makes it possible to circulate part of the material internally for a continued or renewed grinding to obtain the fineness of the product aimed at.
The invention also relates to a method of grinding material in an air-swept tube mill, wherein unground material and air are fed into one end of the mill and are passed through at least one grinding chamber from which the ground material, entrained in the air, is carried through an internal by-pass through a second or last grinding chamber into a discharge chamber at the other end of the mill, oversize material being caused to pass back from the discharge chamber through the second or last grinding chamber for further grinding and then being entrained by the air flowing into the by-pass again.
The material treated in a mill according to the invention may thus be circulated inside the mill without the use of complicated conveying installations. Thus, a separation of the material treated in the first part of the mill may, for instance, be performed in the discharge chamber, for example, by means of a deflector which deflects the air passing through the chamber so that the coarse particles are separated off in the discharge chamber and subsequently fed to and ground in the second or last grinding chamber.
Preferably, a central tube through the second or last grinding chamber constitutes the by-pass. The air or gas passing through the mill then passes centrally through the second or last grinding chamber without being deflected during this passage. The second or last grinding chamber is arranged concentric with the central tube so that its grinding performance, as regards peripheral speed in relation to the critical speed, is the same as for the other grinding chamber or chambers. Further, the feeding of this grinding chamber may be effected from the discharge compartment by means of conventional lifters and the discharge from the grinding chamber may also be effected by means of a conventional device of scoops which may lift and discharge the material into the air or gas current through the central tube.
The tube mill may include a drying compartment at the inlet end of the mill together with means for feeding hot gas through the mill. The air or gas utilized in the mill thereby serves the dual purpose of drying the material and conveying the ground or semi-ground material through the mill and out of it.
In one example, especially in the construction having a central tube, the tube mill includes an air separator unit mounted co-axially with the mill and arranged to treat the gas with entrained material passing through the by-pass and means for returning oversize material to the discharge chamber.
It has been known to combine a conventional tube mill with an air separator in this manner, but such installations have not been adopted to any appreciable extent because of the complications involved when returning the oversize material to a grinding stage for re-grinding whether this is done by means of external conveyors or internal conveyors, such as a screw conveyor or other equipment. In one design according to the invention the conveyance of the oversize particles does not cause any problems because the oversize material is returned from the discharge chamber directly to the second or last grinding chamber.
The applicability and the efficiency of a mill according to the invention is further improved if the air separator is built into and associated with the mill and has a separate driving mechanism outside the mill.
The air separator preferably forms part of the discharge chamber so that the walls of same constitute the housing for the air separator from which the over-size coarse material is led directly into the last or second grinding compartment by means of a conventional scoop device, whereas the air or gas conveys the final ground product which has passed through the air separator out of the installation. The final material may then be separated from the air or the gas, for example in a cyclone plant outside the mill.
When the gas fed to the mill for the carrying or drying is a gas from another manufacturing unit, such as waste gas from a rotary kiln plant, the amount of gas obtainable depends upon the production in the rotary kiln and is difficult to adapt straight away to the amount required for the grinding and drying because the amount is often too low. To make up for a deficit in this respect the mill may have one or more openings for the supply of additional air to the separator. The amount of air or gas for the drying and the conveyance of the material may be insufficient for the proper function of the air separator so that additional air may be supplied preferably to the discharge chamber and, in one construction, through a passage arranged in the driving shaft of the air separator.
The opening or openings for supplying "additional air" may be provided with means for controlling the amount of air passing therethrough. Air control of this kind may serve to ensure the supply of a correctly measured amount of air to the air separator and thereby obtain the best possible performance of the separator without influencing a drying process, if arranged in a drying compartment at the inlet end of the mill or without influencing the conveyance of the material through the grinding chambers.